The present invention relates to controls for heating, ventilating, and air-conditioning (HVAC) systems, and more specifically to controls for systems that have multiple HVAC units conditioning a common space.
Modern buildings typically use HVAC systems to control indoor temperature, pressure, ventilation rate, and other variables. In many cases, these buildings have open-plan construction in which an open space is temperature-controlled and humidity-controlled by multiple HVAC units. In some cases, these HVAC units are referred to as unitary systems because the heating, cooling, humidification, and fans reside in a single packaged unit.
One application of unitary equipment is for the cooling and humidity control of data centers. In this application, it is common to have many HVAC units deployed throughout the data center. They are often floor-standing units, but may be wall-mounted, rack-mounted, or ceiling-mounted. The HVAC units often provide cooled air either to a raised-floor plenum, to a network of air ducts, or to the open air of the data center. The data center itself, or a large section of a large data center, typically has an open-plan construction, i.e. no permanent partitions separating the air in one part of the data center from the air in another part.
Another application of unitary equipment is for the cooling and humidity control of commercial buildings such as retail stores. In this application, it is common to have many HVAC units deployed on the rooftop of the building. The HVAC units typically provide cooled air either to a network of air ducts or directly into the open area of the store. The building itself, or large sections of it, typically has an open-plan construction, with no permanent partitions separating the air in one part of the building from the air in another part.
HVAC units for open-plan spaces such as data centers or retail stores are typically operated with decentralized, stand-alone controls. In the case of the data center HVAC units, it is common for each unit to operate in an attempt to control the temperature and humidity of the air entering the unit from the data center. For example, an HVAC unit may contain a sensor that determines the temperature and humidity of the air entering the unit. Based on the measurements of this sensor, the controls of that HVAC will alter operation of the unit in an attempt to change the temperature and humidity of the air entering the unit to align with the set points for that unit.
Since it is common for these units to be located in close proximity, this operating strategy often causes one unit to fight against another, resulting in oscillating temperature and humidity. In some cases, one unit may be heating while another unit is cooling, thus causing a waste by simultaneously heating and cooling. This problem is compounded by sensor errors from unit to unit, by the fact that the units may be configured with different set points (e.g. temperature that is to be maintained), and by the fact that humidity control is coupled with temperature control.
In a retail store or other commercial building with rooftop units and open-plan construction, the situation is similar. The units are operated with standalone controls where each unit attempts to control the temperature measured by one sensor located in the store. Since the open-plan construction allows free flow of air between sensors, the operation of one unit is coupled to the operation of another unit. Problems such as close sensor or unit location, sensor errors, and differing setpoints for the HVAC units can cause instability and unit-to-unit heating and cooling at the same time.
An alternative approach for data centers is to control each unit based on a sensor located in the data center instead of a single sensor located in the unit. U.S. patent application 2006/0206291 A1 to Bash et al. describes a method for assigning multiple sensors in an open-plan data center to a set of actuators, where there are typically fewer actuators than sensors. Each actuator is controlled (i.e. whether to heat or cool) based on one control sensor that is chosen from a set of sensors assigned to that actuator. Although Bash provides options for selecting the one control sensor, similar problems of close sensor or unit location, sensor errors, and differing setpoints can cause instability and unit-to-unit heating and cooling at the same time.
Another problem with decentralized control of HVAC units in data centers, retail stores, and other open-plan buildings is that the control is not fault tolerant. If one HVAC unit or one of its final control elements fails, or if the HVAC unit is intentionally brought off-line, the controls of the other units have no way to compensate for the failed or stopped unit.
Therefore, it is desirable to provide methods and systems that account for the coupling between units and provide mechanisms for fault tolerance.